1. FIELD OF THE INVENTION
The present invention relates to a single-polarization optical fiber (absolute single-polarization optical fiber) which is required as a transmission medium in the coherent light transmission system, a transmission medium for the ultra-high-speed transmission system or a coupler between optical circuit elements with the polarization characteristic so that only one polarization can be propagated and more particularly a single-polarization optical fiber which persists to propagate only one of two orthogonally polarized polarizations while suppressing the other polarization by increasing its transmission loss.
2. DESCRIPTION OF THE PRIOR ART
Single-polarization optical fibers so far proposed have the cross sectional constructions as shown in FIGS. 1 and 3A, respectively. The construction as shown in FIG. 1 is disclosed in "A Single-Polarization Fiber", J. R. Simpson et al., IEEE Lightwave Technology, LT-1, No. 2, pp. 370-374, 1983, "High-Birefringence Optical Fibers by Preform Deformation", R. H Stolen et al., ibid., LT-2, No. 5, pp. 639-641, 1984, and U.S. Pat. Nos. 4,515,436 and 4,529,426. That is, in the case of the single-polarization optical fiber shown in FIG. 1, an inner elliptical cladding 2 is disposed to surround a core 1 and an outer elliptical cladding 3 is disposed to surround both the core 1 and the inner cladding 2. However, such optical fiber is not circular in cross section so that when it is welded to a single-mode optical fiber circular in cross section, its rectangular cross section becomes again a circular cross section and consequently the elliptical cross section of the inner cladding 2 also becomes almost a circular cross section whereby the birefringence is decreased. When one polarization is guided into the optical fiber, the other polarization component is generated during the propagation. The ratio in power between such two polarizations is called "crosstalk" and the lesser the crosstalk, the higher the characteristic become. A degree of crosstalk is increased with increase of the ellipticity of the inner cladding 2. In the case of the fabrication of the conventional single-polarization optical fiber of the type shown in FIG. 1, as shown in FIG. 2, a preform comprising a core 1A, an inner cladding 2A and an outer cladding 3A is heated and squeezed from both sides by squeezing members W. The preform thus flattened is further drawn into an optical fiber so that the probability that the cross section of the inner cladding is reproduced as desired is low and furthermore the shape in the longitudinal direction of the preform is varied. As a result, it is difficult to attain a low loss and a low crosstalk as well throughout a long optical fiber line.
FIG. 3A shows a cross section of another conventional single-polarization optical fiber disclosed in Japanese Patent Application Laid-Open Nos. 61-200,509 and 61-185,703. Neither of such patent applications discloses the refractive index of the stress-applying portion. FIGS. 3B and 3C illustrate the refractive index profiles in the x- and y-directions which are estimated from the whole description of the above-mentioned patent applications.
The single-polarization optical fiber shown in FIG. 3A is a double-cladding type single-mode optical fiber which includes of a core 1, an inner cladding 4 and an outer cladding 5 and has the W-type refractive index profile and in which stress-applying portions 6 which have a thermal expansion coefficient higher than those of the inner and outer claddings 4 and 5 are added on the x-axis so that a stress is applied to the core 1 in x-direction and hence the core 1 has a high birefringence, whereby one of HE.sub.11.sup.x mode (x-polarization) and HE.sub.11.sup.y mode (y-polarization) is cutoff while the other is permitted to propagate through the optical fiber. However, in this construction, only the stress distribution is taken into consideration and the anisotropy of the refractive index distribution is not taken account. With above-described construction, one mode is cutoff by the outer cladding so that the outer cladding must be arranged considerably closely to the core. As a result, an almost all the portion of each stress-applying portion must be located within the cuter cladding. When the refractive index n.sub.3 of the stress-applying portions is higher than the refractive index n.sub.2 of the outer cladding, light is confined in the stress-applying portions and therefore the transmission characteristics are adversely affected. It follows, therefore, that the refractive index n.sub.3 of the stress-applying portions must be made equal to or lower than the refractive index n.sub.2 of the outer cladding. When the refractive index of the stress-applying portions is equal to that of the outer cladding, the effect of the refractive index of the stress-applying portions is masked by the refractive index of the outer cladding so that the birefringence due to the anisotropy of the refractive index distribution (geometical birefringence) is considerably decreased. On the other hand, when the refractive index n.sub.3 of the stress-applying portions is made lower than the refractive index n.sub.2 of the outer cladding, the birefringence due to stresses (stress-induced birefringence) and the geometrical birefringence are cancelled by each other so that the total birefringence which represents the sum of the birefringence due to stresses and the birefringence due to the geometrical shape becomes lower than the birefringence only due to the existence of stresses.
As described above, in the cases of the conventional single-polarization optical fibers, it is difficult to decrease losses and crosstalk and there exists a defect that the single-polarization wavelength band is narrow.